The invention relates to vacuum equipment for a fiber web machine, the vacuum equipment including a frame arranged to be supported to the fiber web machine, and a wearing construction adapted to the frame and arranged partially open on the surface by means of several openings for extending a vacuum effect out from within the frame and further to a fabric included in the fiber web machine and adapted in contact with the wearing construction. The invention also relates to a fiber web machine provided with vacuum equipment.
Vacuum apparatuses are used in a fiber web machine for various purposes. Most common of these are so called vacuum boxes which are used to remove water from the web produced for increasing the dry content. One vacuum box application is referred to as a felt suction box, which is used in the press section of a fiber web machine. In the press section, it is also possible to use a so called transfer suction box, which ensures detachment of the web at a correct time when transferred from a fabric to another. Furthermore, vacuum suction boxes are used in fabric re-conditioners to absorb the cleaning liquid sprayed to the fabric together with the impurities. Vacuum equipment are also present in the forming section. In all applications the vacuum equipment includes a frame extending from one side of the fiber web machine to the other, over the entire fabric width. The frame additionally includes a wearing construction set in contact with the fabric. Furthermore, the wearing construction is open for its surface for extending the vacuum effect generated within the frame to the fabric. It is strived to adapt the wearing construction in such a way that it is resistant in use without excessively wearing the fabric and without wearing itself.
The wearing construction can be formed of several successive blades adapted at a distance from each other. That is, openings are formed by the slits between the blades. Conventionally, the wearing parts of the blades are made of a ceramic material, whereby the construction becomes expensive and sensitive to damage. During use, the fabric, or a felt in case of a felt suction box, is drawn to the slit due to the suction effect generated by the vacuum. This causes friction, which further increases the energy consumption. In addition, the fabric wears disadvantageously fast. For example, with two slits of 15 mm, the efficient dewatering area achieved is about 300 cm2 per length meter. Here the term ‘length’ refers to the dimension of the vacuum equipment in the transverse direction of the fiber web machine. In practice, it is impossible to increase the slit width due to the fabric wear and increased energy consumption. A sufficient dewatering efficiency has required high vacuum levels, which leads to high operating costs.
Attempts have been made to replace the blades with a wearing construction in which the openings are composed of several holes. Such holes have been machined to a thick solid material. In this case the wearing construction becomes expensive, but a larger dewatering area is achieved with perforated holes compared to blades, without increased felt constriction. As the holes are relatively small, fabric constriction can be avoided. The dewatering time also increases, which makes dewatering more efficient. At the same time, low vacuum levels can be used, which reduces fabric constriction. Then the friction is low resulting in slow fabric wear and a reduced effect of the vacuum equipment on the driving power. In practice, one vacuum apparatus equipped with perforated holes can remove more water than two apparatuses with slit openings. However, a machined wearing construction is expensive, and such long, yet small, holes get gradually plugged. In practice, the holes must be regularly cleaned, which increases production breaks. In addition, changing a blade construction into a hole construction is difficult, often even impossible. Fabric wear may even increase in some cases.